The preparation of polydienes, e.g. cis-polybutadiene (BR), on the basis of metal-organic Ziegler-Natta catalysts is a process which has been used on the industrial scale for a long time. The commercially available grades are characterized by different microstructures. The high-cis grades have cis contents of over 90% and vinyl contents of up to 4%: Nd--BR (97% cis, 2% trans, 1% vinyl), Ni--BR (96% cis, 2% trans, 2% vinyl), Co--BR (95% cis, 3% trans, 2% vinyl), Ti--BR (92% cis, 4% trans, 4% vinyl) (Ullmanns Encyklopadie der technischen Chemie (Ullmann's Encyclopaedia of Chemical Technology), Verlag Chemie, Weinheim, 4th edition, volume 13, pages 602-604; "Handbuch fur die Gummi-Industrie" ("Handbook for the Rubber Industry"), Bayer AG, 2nd edition, chapter A8.1).
Li--BR, on the other hand, is prepared by an anionic process using lithium alkyl catalysts. The trans content exceeds the cis content in this case (35% cis, 55% trans, 10% vinyl).
It is further known that high-cis diene rubbers with vinyl contents of &gt;10% can be prepared using metal-organic catalyst systems, especially metallocenes, e.g. cyclo-pentadienyltitanium trichloride (CpTiCl.sub.3)/methylaluminoxane (MAO) (L. Oliva, P. Longo, A. Grassi, P. Ammendola, C. Pellecchia, MaKromol. Chem., Rapid Commun. 11 (1990) 519-524) or cyclopentadienyltributoxytitanium/MAO (G. Ricci, L. Porri, A. Giarrusso, Macromol. Symp. 89 (1995) 383-392).
It is also known to polymerize conjugated dienes in the liquid monomers without the addition of solvents. However, such a process has the disadvantage that complete polymerization is accompanied by the evolution of a large quantity of heat, which is difficult to regulate and is therefore potentially hazardous. Moreover, this process also causes environmental pollution when the polymers are separated from the monomers.
In recent years the vapour phase process has proved particularly advantageous especially for the preparation of polyethylenes and polypropylenes and has achieved industrial success. The environmentally relevant advantages of the vapour phase process are based especially on the fact that no solvents are used and emissions and effluent pollution can be reduced.
EP 647 657 discloses a catalyst system which polymerizes butadiene to very high-cis polybutadiene in the vapour phase. It is further known that a system consisting of CpTiCl.sub.3 and MAO is capable of polymerizing butadiene without a solvent (WO 96/04322).
The main areas of application for polybutadiene are tyre manufacture, industrial rubber goods and the modification of plastics.
In tyre manufacture, it is known that the various components, such as tread, side wall, steel belt plies, carcass and heel, are made into the blank and then vulcanized. A high cis content therefore has a positive effect in tyre manufacture because of the compounding adhesiveness and unvulcanized strength ("Kunststoffe und Elastomere in Kraftfahrzeugen" ("Plastics and Elastomers in Motor Vehicles"), G. Walter, Verlag W. Kohlhammer Stuttgart, Berlin, Cologne, Mainz, 1985, chapter 4.7.17; "Handbuch fur die Gummi-Industrie" ("Handbook for the Rubber Industry"), Bayer A G, 2nd edition, chapter A8.1).
On the other hand it is known that increasing the vinyl content improves certain properties of the tyre, especially the wet grip. Improving the wet grip ensures greater safety on the road.
For conventional tread compounds, however, an improvement in wet grip is accompanied by a decrease in rolling resistance and hence an increase in motor vehicle fuel consumption and emissions. It has been found that the rolling resistance can be correlated well with the loss factor tan .delta. recorded at a frequency of 10 Hz and a temperature of 60.degree. C., a drop in the loss factor at 60.degree. C. being accompanied by a decrease in rolling resistance (K. H. Nordsiek, Kautschuk, Gummi, Kunststoffe 39 (1986) 599-611; R. Bond, G. F. Morton, L. H. Krol, Polymer 25 (1984) 132-140).
It is known that tyre properties can be adjusted by compounding various types of synthetic rubber. However, this process is expensive and the problem of phase separation can arise during compounding.